Scientific Communities Striving for a Common Cause: Innovations in Carbon Cycle Science

Where does the carbon released by burning fossil fuels go? Currently, ocean and land systems remove about half of the CO₂ emitted by human activities; the remainder stays in the atmosphere. These removal processes are sensitive to feedbacks in the energy, carbon, and water cycles that will change in the future. Observing how much carbon is taken up on land through photosynthesis is complicated because carbon is simultaneously respired by plants, animals, and microbes. Global observations from satellites and air samples suggest that natural ecosystems take up about as much CO₂ as they emit. To match the data, our land models generate imaginary Earths where carbon uptake and respiration are roughly balanced, but the absolute quantities of carbon being exchanged vary widely. Getting the magnitude of the flux is essential to make sure our models are capturing the right pattern for the right reasons. Combining two cutting-edge tools, carbonyl sulfide (OCS) and solar-induced fluorescence (SIF), will help develop an independent answer of how much carbon is being taken up by global ecosystems. Photosynthesis requires CO₂, light, and water. OCS provides a spatially and temporally integrated picture of the “front door” of photosynthesis, proportional to CO₂ uptake and water loss through plant stomata. SIF provides a high-resolution snapshot of the “side door,” scaling with the light captured by leaves. These two independent pieces of information help us understand plant water and carbon exchange. A coordinated effort to generate SIF and OCS data through satellite, airborne, and ground observations will improve our process-based models to predict how these cycles will change in the future

Example Data and Code from SI

The example code and relevant data provided in the manuscript Supplemental Information, including SIData S1, S2, S3 and S4

Data from: Local range boundaries versus large-scale tradeoffs: climatic and competitive constraints on tree growth

Species often respond to human‐caused climate change by shifting where they occur on the landscape. To anticipate these shifts, we need to understand the forces that determine where species currently occur. We tested whether a long‐hypothesised trade‐off between climate and competitive constraints explains where tree species grow on mountain slopes. Using tree rings, we reconstructed growth sensitivity to climate and competition in range centre and range margin tree populations in three climatically distinct regions. We found that climate often constrains growth at environmentally harsh elevational range boundaries, and that climatic and competitive constraints trade‐off at large spatial scales. However, there was less evidence that competition consistently constrained growth at benign elevational range boundaries; thus, local‐scale climate‐competition trade‐offs were infrequent. Our work underscores the difficulty of predicting local‐scale range dynamics, but suggests that the constraints on tree performance at a large‐scale (e.g. latitudinal) may be predicted from ecological theory

Raw_Ring_Widths_012118

Raw ring width data (2013-variable start date) for focal trees from nine species-replicates. "avRingWidth_Metadata_VERSION##.csv" contains metadata for data files, including column descriptions

README

README for Dryad repository

Tree_Info_BAI_012118

Focal tree size, competitive environment, regeneration counts within 5m of focal tree, and focal tree 10yr Basal Area Increment (2003-2012) data for 9 species replicates. One .csv per species replicate, named by Transect ID (CO/MT/WA) and Species ID (PIPO = Pinus ponderosa, POTR = Populus tremuloides, ABLA=Abies lasiocarpa, TSHE = Tsuga heterophylla, PSME = Psuedotsuga menziesii). treeinfobai_Metadata_VERSION# .csv has metadata including column descriptions

Loss of whole-tree hydraulic conductance during severe drought and multi-year forest die-off

Understanding the pathways through which drought stress kills woody vegetation can improve projections of the impacts of climate change on ecosystems and carbon-cycle feedbacks. Continuous in situ measurements of whole trees during drought and as trees die hold promise to illuminate physiological pathways but are relatively rare. We monitored leaf characteristics, water use efficiency, water potentials, branch hydraulic conductivity, soil moisture, meteorological variables, and sap flux on mature healthy and sudden aspen decline-affected (SAD) trembling aspen (Populus tremuloides) ramets over two growing seasons, including a severe summer drought. We calculated daily estimates of whole-ramet hydraulic conductance and modeled whole-ramet assimilation. Healthy ramets experienced rapid declines of whole-ramet conductance during the severe drought, providing an analog for what likely occurred during the previous drought that induced SAD. Even in wetter periods, SAD-affected ramets exhibited fivefold lower whole-ramet hydraulic conductance and sevenfold lower assimilation than counterpart healthy ramets, mediated by changes in leaf area, water use efficiency, and embolism. Extant differences between healthy and SAD ramets reveal that ongoing multi-year forest die-off is primarily driven by loss of whole-ramet hydraulic capability, which in turn limits assimilation capacity. Branch-level measurements largely captured whole-plant hydraulic limitations during drought and mortality, but whole-plant measurements revealed a potential role of other losses in the hydraulic continuum. Our results highlight the importance of a whole-tree perspective in assessing physiological pathways to tree mortality and indicate that the effects of mortality on these forests’ assimilation and productivity are larger than expected based on canopy leaf area differences

Dry-season canopy water content maps for California vegetation from 1990-2017, link to GeoTiffs

Dry-season canopy water content maps for California vegetation from 1990-2017. Each of the 27 individual files (1.9 GB each) corresponds to the July-August time period of the designated year, stored as a GeoTiff with LZW compression and a -9999 nodata value. Units are mL water per square meter. Each dataset is in UTM 10 N (EPSG 32610), with a 30 m ground-level spatial resolution

Terrestrial Gross Primary Production: Using NIRv to Scale from Site to Globe

Terrestrial photosynthesis is the largest and one of the most uncertain fluxes in the global carbon cycle. We find that NIRv, a remotely sensed measure of canopy structure, accurately predicts photosynthesis at FLUXNET validation sites at monthly to annual timescales (R2 = 0.68), without the need for difficult to acquire information about environmental factors that constrain photosynthesis at short timescales. Scaling the relationship between GPP and NIRv from FLUXNET eddy covariance sites, we estimate global annual terrestrial photosynthesis to be 147 Pg C y-1 (95% credible interval 131-163 Pg C y-1), which falls between bottom-up GPP estimates and the top-down global constraint on GPP from oxygen isotopes. NIRv-derived estimates of GPP are systematically higher than existing bottom-up estimates, especially throughout the mid-latitudes. Progress in improving estimated GPP from NIRv can come from improved cloud-screening in satellite data and increased resolution of vegetation characteristics, especially photosynthetic pathway

Site- and Species-Specific Influences on Sub-Alpine Conifer Growth in Mt. Rainier National Park, USA

Identifying the factors that influence the climate sensitivity of treeline species is critical to understanding carbon sequestration, forest dynamics, and conservation in high elevation forest/meadow ecotones. Using tree cores from four sub-alpine conifer species collected from three sides of Mt. Rainier, WA, USA, we investigated the influences of species identity and sites with different local climates on radial growth–climate relationships. We created chronologies for each species at each site, determined influential plant-relevant annual and seasonal climatic variables influencing growth, and investigated how the strength of climate sensitivity varied across species and location. Overall, similar climate variables constrained growth on all three sides of the mountain for each of the four study species. Summer warmth positively influenced radial growth, whereas snow, spring warmth, previous summer warmth, and spring humidity negatively influenced growth. We discovered only a few subtle differences in the climate sensitivity of co-occurring species at the same site and between the same species at different sites in pairwise comparisons. A model including species by climate interactions provided the best balance between parsimony and fit, but did not lead to substantially greater predictive power relative to a model without site or species interactions. Our results imply that at treeline in moist temperate regions like Mt. Rainier, the same climatic variables drive annual variation in growth across species and locations, despite species differences in physiology and site differences in mean climates